Vibration control of ship Vibration & Noise R&D Ship and Ocean R&D Institute 2016.10.27

Vibration measurement Ship vibration control Conclusion Contents Introduction Vibration criteria Initial prediction Vibration analysis Global vibration analysis Local vibration analysis Vibration measurement Ship vibration control Conclusion

Introduction Process of ship vibration control to avoid excessive vibration Select propeller blades, main engine cylinder and M/C Calculated natural frequency and vibration response Measured vibration response

Criteria for human beings Vibration criteria Criteria for human beings Ensuring comfort and well-being. The international standard ISO 6954(2000)[1] Evaluation of vibration w.r.t habitability on passenger and merchant ships Structural vibration the low risk of fatigue cracks VIBRATION CLASS(2011) Equipment protect the machinery from the excessive vibration internal sources(machinery itself) and the external sources(from other machinery) Building Specifications International standards class recommendation Vibration criteria

Vibration criteria for human being Overall frequency-weighted r.m.s. values Frequency range : 1 Hz to 80 Hz Area classification A Passenger cabins B Crew accommodation C Working areas Acceleration (mm/s2) Velocity (mm/s) Velocity (mm/s) Values above which adverse comments are probable 143 4 214 6 286 8 Values below which adverse comments are not probable 71.5 2 107 3 NOTE : The zone between upper and lower values reflects the shipboard vibration environment commonly experienced and accepted.

Criteria for the structural vibration r.m.s vibration r.m.s. values 4 Hz to 200 Hz low risk for fatigue cracks REF : DNV VIBRATION CLASS(2011)

Criteria for the machinery vibration

To check the possibility of resonance Initial prediction To check the possibility of resonance Hull girder natural frequency and 2nd order moment of main engine To decide installation of Moment compensator Wheel house natural frequency and main excitation source To select No. of Propeller blade and Main Engine cylinder

Initial prediction 2 node 3 node 4 node Natural frequencies of hull girder vertical 2 node mode (ref. Jung and Todd) 2 node 3 node 4 node N2v, L, B and D are the natural frequency of vertical 2 node, length(m), breadth(m) and depth(m) High order natural frequency

Initial prediction Mode shape Compensation force Lf La Fm : compensation force Fa : Aft force due to 2nd order moment of main engine M : 2nd order moment of main engine La : distance between main engine aft and nodal point Lf : distance between main engine forward and nodal point L : length of main engine Compensation force Lf La

Mode shape of super structure Initial prediction Natural frequencies of Super structure(ref. JH Park ) Mode shape of super structure   (Hz) H:Height(m) of Super structure L:length(m) B: breadth(m) K:Stiffness of foundation (N/m) M: Mass (ton) α, β, γ : Coefficient

Vibration Analysis Global vibration analysis To investigate the overall vibration characteristics of whole ship Resonance check Free vibration analysis of hull girder including deckhouse, aft body and engine room Response check Forced vibration analysis to calculate the vibration responses of selected points representing overall vibration behaviors Excitation force Propeller fluctuation pressure Engine H and X moment Three dimensional finite element model Mode shapes of a ship Calculated vibration response at deck house top

Vibration Analysis Local vibration analysis To avoid the resonance Design target frequency 10% higher than a excitation frequency Excitation frequency of fluctuation pressure on hull surface due to propeller Main engine external extraction forces Analysis area aft body, engine room and deckhouse area modification works increase of a plate thickness, stiffener size and or girder size based on a concerned mode shape Areas for local vibration evaluation Ex) Propeller 4 blade M/E 6 cylinder NCR: 60, MCR: 70 10.3Hz Design target freq. MCR +10% 4.4 Hz NCR -10% Life boat davit Heli-deck Thruster motor

Vibration measurement To confirm accordance with the criteria global vibration measurement to check global vibration characteristics of whole ship to measure vibration level with increase of RPM by using fixed monitoring system Local vibration measurement To check vibration in cabin w.r.t habitability To check vibration levels of local structures wrt structure failure. To check vibration level of machinerys and outfittings in the deckhouse, engine room and aft body in viewpoint of machinery malfunction by using portable analyzer Typical measurement positions of global vibration Ref: ISO 20283-2:2008[5]

Vibration measurement To confirm a measured vibration level below the limit Waterfall plot to find out the natural frequency of hull structure vibration level is distinguishably high at around the natural frequency Peak plot The slice of waterfall along the each excitation force the vibration level due to each excitation source Waterfall plot Peak plot

Vibration control measure Moment compensator Application areas Excessive vibration same magnitude anti-phase The most effective and widely used method to reduce the vibration is to reduce the excitation force Vibration cancellation Resultant vibration compensated by the control force having same magnitude and opposite phase to the existing force. Vibration control

Vibration control measure Top bracing If source of vibration is main engine, then we can consider top bracing with on-off control The main purpose of top bracing for main engine is to reduce engine vibration. The top bracing is installed on the side of engine.

Vibration control measure the vibration can be controlled by using the on-off control of hydraulic top bracing The vibration level of deckhouse can be keep under the limit by controlling top bracing active(on) below 89 rpm(blue line) and top bracing inactive(off) above 89 rpm(blue line) Vibration reduction by using on-off control at deckhouse T/B on T/B off Vibration limit Velocity (mm/s) RPM of Main Engine

Vibration control measure Modification of structure The simplest and most common way to reduce a vibration increasing the stiffness or supporting the structure the vibration behavior of hull structure shall be figured out by measuring vibration considering the phase of supporting structure Deformed shape Mode shape Modification Vibration reduction

Conclusion In this paper, the process of ship vibration control is explained to avoid excessive vibration from design stage to sea trial. Vibration criteria have to be set up based on the Building Specifications. Initial design stage : predominant excitation sources are selected by using the data of experienced ships such as numbers of main engine cylinders and propeller blades Detail design stage : global vibration and local vibration analysis are carried out by using the Finite Element Analysis Measurements are carried out during the sea trial to assess the vibration performance of ship. If they are not satisfied the criteria, appropriate methods for vibration control are applied.